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Experts have long known that the brain has the ability to suspend the pain response in times of injury and great stress, even after traumatic incidents such as gunshot wounds.

Now, a new study in rats suggests marijuana-like neurochemicals called endocannabinoids may be key to this process.

The discovery may lead to a new class of painkillers with fewer side effects than existing pain medications, report researchers at the University of California, Irvine ( UCI ). Their study appears in the June 23 issue of Nature.

"This study shows for the first time that natural marijuana-like chemicals in the brain have a link to pain suppression," researcher Daniele Piomelli, a professor of pharmacology and director of the Center for Drug Discovery at the UCI School of Medicine, said in a prepared statement.

Stress can provide a delayed pain reaction in certain situations, an effect called stress-induced analgesia. Previous research has identified two kinds of stress-induced analgesia mechanisms in the body -- opioid and non-opioid. This study is the first to offer evidence that the non-opioid form is produced by cannabinoid compounds.

"If we design chemicals that can tweak the levels of these cannabinoid compounds in the brain, we might be able to boost their normal effects," Piomeilli explained.

"Aside from identifying an important function of these compounds, it provides a template for a new class of pain medications that can possibly replace others shown to have acute side effects," he added.

Wounded soldiers and badly injured athletes often report they don't feel pain for hours after being hurt, and now researchers are beginning to understand why - it's because the brain produces its own natural marijuana.

Scientists already knew that pot can reduce some kinds of pain, and that the brain actually produces marijuana-like chemicals called cannabinoids.

But now a team led by a University of Georgia researcher has found that those pot-like substances are crucial in blocking pain in the process called "stress-induced analgesia," according to a study published Wednesday in the journal Nature.

"The reason marijuana produces its effects is because the active ingredient, Delta-9 THC, acts on the same receptors as the brain's natural cannabinoids," said Andrea Hohmann, a researcher in UGA's psychology department and the lead author of the paper.

Their discoveries could mean an eventual end to the debate over whether medical uses of marijuana should be legal: The researchers hope to develop a prescription drug that will boost the body's own marijuana-like chemicals, but one without unwanted side effects - one that won't get you high.

Several states allow doctors to prescribe marijuana for patients with cancer, AIDS or other illnesses, though it's forbidden by federal law.

Pot is not only an effective pain reliever for them, but also suppresses nausea and increases appetite. Nausea and loss of appetite are frequent, unwanted side effects of "opiate" pain-killers like morphine, which act through a different mechanism than the cannabinoids and can have toxic side effectsmarijuana doesn't, Hohmann said.

New drugs wouldn't exactly be pot in a pill, though, but a medicine that would increase the brain's own natural pot-like chemicals, Hohmann said.

Scientists have known about these brain-produced cannabinoids for years, but the new research gets scientists much closer to understanding how they work inside the brain, Hohmann said.

"On a very basic level we have a new understanding of brain control of neural functions. We know that we can basically tweak the levels of the brain's own cannabinoid-like substances," she said.

The research started five years ago with a summer research project in Hohmann's lab by an undergraduate UGA honors student, Mark Neely. Other contributors include students Richard Suplita and Nathan Bolton, UGA psychology faculty members Philip Holmes and Jonathon Crystal and collaborators at the University of California-Irvine, Brown University and two Italian universities, the University of Urbino Carlo Bo and the University of Parma.

Their first step was to show that blocking brain receptors the cannabinoid chemicals act upon also reduced the brain-induced analgesia.

To measure the effect, experimenters shocked rats' feet with an electric current, then timed how quickly the rats flicked their tails away from a heated metal plate. The procedure does not injure the rats but mimics the same stress-induced analgesia seen in humans and animals following serious injuries, Hohmann said.

Since that first experiment, Hohmann and her fellow researchers have identified ways to use synthetic chemicals to increase cannabinoid levels in lab rats' brains. They have also gotten a better understanding of where the effect occurs in the brain.

One of the cannabinoid-enhancing chemicals, discovered by a team led by University of California-Irvine researcher Daniele Piomelli, is the most likely candidate to develop into a prescription drug.

It's possible a drug also would be effective against stress-related disorders such as post-traumatic stress syndrome, Hohmann said.

The discovery is, in a way, similar to earlier finds that animal brains naturally produce chemicals similar to morphine and other opiate pain-killers, she said.

"This is the first time we have data that indicate that the brain's 2AG (one of the pot-like chemicals) suppresses pain. That could have tremendous clinical importance and may lead to new ways to treat pain and stress,"